1. Field of the Invention
The present invention relates to a wavelength conversion light source apparatus and a wavelength conversion method, and for example, relates to a light source apparatus which emits light in the ultraviolet region, such as illumination light used for inspecting a pattern defect of a target object in manufacturing semiconductors and to a wavelength conversion method.
2. Description of Related Art
In recent years, with high integration and large capacity of large scale integrated (LSI) circuits, the line width (critical dimension) required for circuits of a semiconductor element is becoming narrower and narrower. The semiconductor element is manufactured by exposing (transferring) a pattern onto a wafer to form a circuit by a reduced projection exposure apparatus, known as a stepper, while using an original or “master” pattern with a circuit pattern formed thereon. (The original pattern is also called a mask or a reticle, and hereinafter generically referred to as a mask). Therefore, in manufacturing a mask for transferring such a fine circuit pattern onto a wafer, a pattern writing apparatus capable of writing or “drawing” fine circuit patterns needs to be employed. Pattern circuits may be written directly onto a wafer by the pattern writing apparatus. A pattern writing apparatus uses electron beams or laser beams for writing is under development.
Since the LSI manufacturing requires a tremendous amount of manufacturing cost, it is crucial to improve its yield. However, as represented by a 1 gigabit DRAM (Dynamic Random Access Memory), the order of a pattern constituting an LSI has been changing from submicron to nanometer dimensions. One of major factors that decrease the yield of the LSI manufacturing is a pattern defect of a mask used when exposing (transferring) a fine pattern onto a semiconductor wafer by the photolithography technology. In recent years, with miniaturization of an LSI pattern formed on a semiconductor wafer, dimensions of defects to be detected have become extremely small. Thus, a pattern inspection apparatus for inspecting defects of a mask for exposure used in manufacturing LSI needs to be highly accurate.
Meanwhile, with development of multimedia technology, the size of Liquid Crystal Display (LCD) substrates is becoming larger, e.g. 500 mm×600 mm or greater, and the size of a pattern such as a Thin Film Transistor (TFT) or the like formed on the liquid crystal substrate is becoming finer. Therefore, it is increasingly required that an extremely small defect of a pattern should be inspected in a large range. For this reason, development of a pattern inspection apparatus which efficiently and short-timely inspects defects of a pattern of a large area LCD and of a photomask used in manufacturing the large area LCD is urgently required.
As to a conventional pattern inspection apparatus, it is known that an optical image obtained by imaging a pattern formed on a target object or “sample”, such as a lithography mask, is compared with design data or an optical image obtained by imaging an identical pattern on the target object. For example, the following is known as pattern inspection methods: “die to die inspection” method that compares data of optical images of identical patterns at different positions on the same mask, and “die to database inspection” method that inputs, into an inspection apparatus, writing data converted from pattern-designed CAD data to a format for input to the writing apparatus when writing a pattern on a mask, generates a reference image based on the input writing data, and compares the generated reference image with an optical image serving as measurement data obtained by capturing an image of the pattern. According to the inspection method using such inspection apparatus, a target object is placed on a stage so that a light flux may scan the object by the movement of the stage. Specifically, the target object is irradiated with a light flux by an illumination apparatus and an illumination optical system. Light transmitted through the target object or reflected therefrom is focused on a sensor through the optical system. An image captured by the sensor is transmitted as measurement data to a comparison circuit. In the comparison circuit, after position alignment of the images, the optical image and the reference image are compared in accordance with an appropriate algorithm. If the images do not match, it is judged that a pattern defect exists.
As an illumination light of a pattern inspection apparatus which inspects a defect of a fine pattern as described above, it becomes necessary to use light in the ultraviolet region. Then, for generating an ultraviolet light, a wavelength conversion light source apparatus is needed which performs wavelength conversion by letting a fundamental wave pass through a nonlinear crystal to generate a light with a shorter wavelength.
However, if a nonlinear crystal is continuously irradiated with ultraviolet rays for a long time, optical damage will occur on the surface of the nonlinear crystal, and then a wavelength converted output (power of an ultraviolet ray having a converted wavelength) will be reduced. Therefore, when occurrence of optical damage on the nonlinear crystal surface is noticed, conventionally, the position of light irradiated onto the nonlinear crystal is shifted.
FIG. 11 is a schematic diagram showing how to shift an irradiated position concerned. FIG. 11 shows the surface of a nonlinear crystal 80, which is irradiated with a fundamental wave, for converting a wavelength. In the figure, a spot size 82 indicates a size including 86% of the energy of a radiating fundamental wave. A region 84 indicates a size having a diameter twice as long as that of the spot size and including 99.7% of the energy of the radiating fundamental wave. If the fundamental wave continuously irradiates a certain point for a long time, the optical damage mentioned above occurs not only in the spot size 82 but also in the entire region 84 being larger than the spot size. Therefore, even when the fundamental wave passes through the region 84, a wavelength converted output will be reduced.
FIG. 12 shows a relation between a wavelength converted output and time concerned. As mentioned above, when a certain point is continuously irradiated with a fundamental wave for along time, the wavelength converted output will decrease. As shown in the example of FIG. 12, the wavelength converted output gradually decreases as time passes from the irradiation starting. Therefore, as a conventional usage, the whole crystal which is kept in a phasing state is moved in parallel every 10 hours to a position where there is no influence of the deterioration as shown in FIG. 11. Thus, it has been repeated to restore the wavelength converted output by shifting the irradiation position of a fundamental wave every 10 hours, for example, and to continue the irradiation until the surface of the crystal deteriorates. Therefore, as shown in FIG. 12, the output characteristic shows a shape like a saw blade to return the output to the original intensity every 10 hours, for example. The continuous radiation time varies depending upon the type etc. of the nonlinear crystal, and it is acceptable to move the irradiated position every 24 hours, for example.
Conventionally, a method is known in which a transportation means (crystal shifter function) is provided in the mount of the wavelength conversion nonlinear crystal in order to deviate the light path from a deteriorated part of the crystal. For example, a crystal shifter is built in some DUV light source products (for example, refer to a coherent laser catalog in 2001). Further, a proposal has been made to change a light path in the crystal in order to inhibit the change of the output power and the beam shape (refer to, e.g., Japanese Patent Application Laid-Open (JP-A) No. 2000-252570), and however, its substantial function is quite the same as that of the crystal shifter.
When a KTP crystal of 5 square mm, for example, is used as the nonlinear crystal 80, since the edge cannot be used, an effective section is 4 square mm. If specifying the spread angle of the fundamental wave to be 0.5 mrad (full angle) and the focal length of the condenser lens to be 250 mm, the diameter of the spot size on the KTP crystal will be 0.125 mm. For moving the crystal in parallel to a position having no influence of crystal degradation caused by the optical damage mentioned above, it is necessary to perform parallel translation of three times the spot size 82. That is, it is needed to move 0.375 mm every 10 hours. In the effective region of 4 square mm, it is possible to move the crystal to eleven points in lengthwise and crosswise. In the case of all the eleven points being used, if the output is restored every 10 hours as shown in FIG. 11, the crystal has to be exchanged after being continuously used for 1210 hours (for about 50 days).
When the inventors observed the surface of the crystal after using it in the above way, it was found out that there was a trace after being used at each point and an influence layer caused by the degradation had been formed on the surface. Because of this, it was also found out that the crystal has to be exchanged when all the points have been used.
Conventionally, as mentioned above, performance degradation is prevented by avoiding deteriorated points by performing a periodical step movement further larger than a region which is larger than the spot size of a fundamental wave. However, in such an operation, since the fundamental wave radiates the same position for a longtime, a wavelength-converted light is generated, thereby producing optical damage at the position used. Then, a trace of the radiation remains and a poor surface whose influence layer part is larger than the spot size is produced on the crystal surface. Therefore, there is a problem that one continuous radiation makes a broad range unusable and then, in shifting the irradiated position, the region to be used is limited depending upon the area of the surface of the crystal. Moreover, it is also a problem that, because of temporal (time-course) degradation of each position, the output characteristic after shifting changes as time passes.
In addition, as to a nonlinear crystal, there is disclosed a technique in which degradation of a crystal is restored by performing an annealing treatment under certain conditions before incurring optical damage on the surface of the crystal (refer to, e.g., Japanese Patent No. 4109869).